I’m new to the forum and possibly a bit late to this party. Hopefully my 2 penny worth will help rather than hinder, though.
You can tour on any size bike. Though if you’re on limited time to get to, say, Austria, I’d discount anything that can’t cruise all day at 130kph (so a minimum 509cc, I guess).
If you do need to cruise at 130kph all day, don’t do it with a naked bike. Get one with a half decent fairing or else you’re going to be a bit battered at the end of each day.
Pillions are great (is mine watching me type this, I ask). Really handy for paying tolls, saving you having the hassle of taking gloves off, digging the money out, paying, etc. etc. Also, sitting in a bar or eating your evening meal alone is boring after a few days.
We’re not “credit card and a change of underpants” tourers, so we tend to carry a lot of kit. Make sure your luggage capacity is adequate (what to take is a whole different topic). We have around 100 litres of luggage, a mate manages with a tank bag. I always ride upwind of him after a few days on the road.
A personal preference is for a bike with a low CoG and hides its weight well at slow speeds and on the move. The ST1100 was good in those terms, the FJR1300 less so. My current bike, the BMW R1200 GS is also good at that.
As mentioned before, try a few bike. Do not ignore bikes like the 500CC or NC750 Hondas, they are remarkably competent.